Battle Moveset

A Good Moveset for Mightyena would have to be this:

Attacks:

Protect
Torment
Swagger
Poison Fang

Items Attached:

Leftovers OR
Lum Berry

Preferred Nature:

Impish

Strategy Using Mightyena

Mightyena stats aren't the greatest in the game, but Mightyena does learn a few of those annoying moves that may help it out.
Even though its a dark type (Special), it does have good attack, and could possibly be used as an all out of sweeper.
The above moveset allows for you to avoid attacks that might do considerable damage to Mightyena, like Brick break, use Protect if you think your opponent will use it, then Torment to prevent that Pokemon from using the same move within the two consecutive turns.
Swagger is worth the increase in attack strength, as it confuses your opponent, and when they do hit themselves, they recieve more damage too.
Poison fang is a decent move if you want a chance of an effect similar to toxic happen to your opponent. Plus, it will do damage too.
Another moveset that was suggested by Onimisha is: Mightyena@Leftovers: Body Slam (Paralysis), Attract, Swagger, Substitute, which is also pretty effective too.

EV Corner:

Max out the EVs in Defense for Mightyena with any remaining in Special Defense & HP

Strategy Against Mightyena

Mightyena' Defenses are about average but it's Physical is lower so a Physical Volley or a Fighting Volley should work.

Contest Moveset

A good Contest Moveset for Mightyena would have to be this for the Smart Contest best with Calm, Gentle, Careful, Sassy Nature:

Attacks:

Swagger
Poison Fang
Body Slam
Shadow Ball

Items Attached:

Green Scarf

Strategy Using Mightyena

If you do the attacks in this following order you should have very few problems; 1st - Swagger, 2nd - Poison fang, 3rd - Body slam, 4th - Poison Fang, 5th - Shadow ball

Locations in Games

Ruby/Sapphire Evolve Poochyena Colosseum Trade from R/S Fire Red/Leaf Green Trade from R/S

Animé Appearences

Mightyena has had a few Animé Appearences. First there was a pack of them which had a Poochyena in it that Max helped to evolve into a Mightyena to join his friends. After that, Butch of Team Rocket used one in his battle against Richie, Tracey, Misty & Casey. The rest of it's appearences were under the control of Team Magma. Episode 287: A Bite To Remember! Special 10: Pokémon Investigation! Search for Prof. Oak! Episode 303: A Three Team Scheme! Movie 6: Jirachi - Wish Maker Episode 312: The Spheal of Approval! Special 12: The Seriousness of Misty's Match! Risking a Life?!? Episode 330: Once Again! Team Magma VS Team Aqua! Mt. Chimney Battle!